The genus Hydrangea encompasses diverse flowering shrubs and vines known for their substantial, vividly colored bloom clusters. These plants are popular globally due to their showy, long-lasting flowers. Appearance varies widely, ranging from compact, rounded shrubs to towering, tree-like forms and climbing vines. Hydrangeas are generally deciduous, shedding their leaves in the fall, but their woody structure remains throughout the winter.
The Signature Blooms: Flower Forms and Color Variation
The most recognizable feature of the hydrangea is its flower cluster, or inflorescence, which comes in three primary shapes. The popular “mophead” form is characterized by large, densely packed, globe-shaped clusters composed almost entirely of showy, sterile florets. The “lacecap” inflorescence presents as a flatter cluster, featuring a central mass of tiny, fertile flowers surrounded by an outer ring of larger, showier sterile florets. The third major type, the “panicle” or “cone” shape, is elongated and pyramidal.
The color of the blooms adds visual complexity, particularly in the bigleaf hydrangea (Hydrangea macrophylla). This species changes color based on soil chemistry, mediated by aluminum ions. Flowers turn blue in acidic soil (pH 5.0–5.5) where aluminum is readily available. Conversely, blooms are pink or red in alkaline soil (pH 6.0 or higher), where aluminum is less available.
The presence of aluminum ions interacts with the plant’s anthocyanin pigment to produce the blue hue. If the soil pH is between 5.5 and 6.0, the flowers may display shades of purple or a mixture of pink and blue. White-flowered varieties do not possess this pigment and remain white regardless of soil chemistry. Other colors, such as the lime-green or crimson that develop as some panicle flowers age, are stable and not pH-dependent.
Foliage and Overall Plant Structure
While the flowers are the main attraction, the leaves provide important visual cues. Hydrangea leaves are typically large, broad, and oval to ovate, often measuring 4 to 8 inches in length. They are arranged in opposite pairs along the stems and feature distinct, coarsely serrated edges. The texture is usually thick and crisp, with prominent vein patterns.
Most hydrangeas grow as deciduous shrubs, forming a rounded, bushy mound. Stems transition from green or reddish when young to woody and sturdy as they mature, providing necessary support for the large flower heads. Size ranges from compact two-foot varieties to large shrubs reaching six feet or more. A few species, such as the climbing hydrangea, exhibit a vine-like habit, using aerial roots to ascend structures.
Distinguishing the Most Common Types
The Bigleaf Hydrangea (Hydrangea macrophylla) is identified by its large, glossy, elliptical leaves and the classic mophead or lacecap flower forms. Its defining characteristic is the flower color variability, shifting between pink and blue based on soil aluminum availability. This species is often sensitive to cold, making it prone to damage in colder climates.
The Panicle Hydrangea (Hydrangea paniculata) is easily distinguished by its upright, cone-shaped flower clusters, known as panicles. These flowers typically emerge creamy white and gradually deepen to pink or reddish hues later in the season. This species is highly valued for its robust, woody structure, which allows it to be trained into a small tree form. It is also the most cold-tolerant of the common hydrangeas.
The Oakleaf Hydrangea (Hydrangea quercifolia) stands out due to its distinctive foliage, which is deeply lobed, closely resembling oak leaves. It produces elongated, white flower clusters similar in shape to the panicle type. This species offers exceptional fall interest, as its leaves change to shades of bronze, deep red, and purple before dropping.